1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to wireless local area networks (WLANs), and more particularly, to preventing a high throughput (HT) station and an 802.11 legacy station from colliding with each other in a WLAN where HT stations and 802.11 legacy stations coexist.
2. Description of the Related Art
In a wireless local area network (WLAN), Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)-based Medium Access Control (MAC) methods are widely used. CSMA/CA is a type of data transmission method in which, even when no data is currently being transmitted via a network cable, a signal for determining whether data is currently being transmitted via the network cable is transmitted and data is transmitted only after the signal is determined to have been successfully transmitted without collision.
In detail, according to CSMA/CA, a first station detects a sub-carrier indicating whether there is a station currently transmitting data. If there is a station currently transmitting data, the first station stands by for a predetermined amount of time, determines whether there is a sub-carrier transmitted by another station, and begins to transmit data if no sub-carrier is currently being transmitted.
According to CSMA/CA, both a physical carrier sensing method and a virtual carrier sensing method are used at the same time. The physical carrier sensing method is a carrier sensing method in which a physical layer (PHY) determines whether a power of higher than a predefined value has been received and notifies an MAC layer of whether a medium is currently busy or idle based on the results of the determination, and the virtual carrier sensing method is a carrier sensing method in which, if an MAC protocol data unit (MPDU) can be properly extracted from a received Physical Layer Convergence Procedure (PLCP) protocol data unit (PPDU), stations interpret one of a plurality of header fields of the MPDU, i.e., a duration/identifier field of the MPDU, and determine whether a medium is currently busy based on the results of the interpretation. Stations use both the physical carrier sensing method and the virtual carrier sensing method to determine whether a medium is currently busy and do not attempt to access the medium if the medium is determined as being busy.
Referring to FIG. 1A, an MAC header of a data frame which can be transmitted via a typical IEEE 802.11 WLAN includes duration information specifying the time required to receive an acknowledgement (ACK) frame in return for the data frame after the transmission of the data frame. A plurality of stations which receive the data frame interpret the MAC header of the data frame and do not attempt to access a medium during a predetermined time period specified in the MAC header of the data frame. Therefore, the stations can be prevented from colliding with one another. Due to the characteristics of a WLAN, all of a plurality of stations in a WLAN can receive frames regardless of whether the frames are destined for only one of the stations.
FIG. 1B is a diagram illustrating the format of a data frame which is used in a typical IEEE 802.11a network. Referring to FIG. 1B, a signal field of the IEEE 802.11a frame includes rate information (RATE) and length information (LENGTH). Thus, duration information of the data frame can be obtained by analyzing the rate information and the length information of the data frame. Therefore, the virtual carrier sensing method can be realized.
The virtual carrier sensing method can be effectively applied to CSMA/CA only when an MPDU/PHY service data unit (PSDU) can be interpreted properly without any errors, i.e., only when the value of an MAC header of a frame can be read out properly.
When errors occur due to an unstable channel state during the transmission of a frame at a high transmission rate by a transmitting station, or when a receiving station cannot properly handle the high transmission rate, a received MPDU/PSDU cannot be interpreted properly. In this case, the virtual carrier sensing cannot be used, and thus, the performance of CSMA/CA decreases. Therefore, a plurality of hearing stations are highly likely to collide with one another.
An HT station is a station such as a multi-input multi-output (MIMO) station which has better data transmission capabilities than an existing legacy station such as a station based on the IEEE 802.11a/b/g standard.
In order to address the problem of such a high probability of a plurality of stations colliding with one another in a WLAN, a method based on the IEEE 802.11a standard which is currently being introduced has been suggested in which, when a plurality of HT stations and a plurality of legacy stations coexist in a WLAN, as illustrated in FIG. 3, a header having a legacy format (L-Preamble, L-SIG) is used as a PHY header of a frame to be transmitted and duration information which is previously included in an existing MAC header is included in the PHY header of the frame to be transmitted such that the duration information can represent the time required to receive an ACK frame after L-SIG. The duration information will now be referred to as extended PHY protection (EPP) information.
FIG. 3 is a diagram for explaining a related art method of controlling the access of a plurality of stations to a medium by using extended PHY protection (EPP) information. Referring to FIG. 3, a plurality of stations can be prevented from colliding with one another by using the EPP information. However, the method illustrated in FIG. 3 results in unfairness regarding access to the medium among the stations.
Referring to FIG. 3, a plurality of legacy stations can interpret a PHY header of a data frame having an HT format. However, the legacy stations cannot interpret the remaining portions of the data frame, thus causing an error. Then a PHY layer (i.e., a baseband layer) notifies an MAC layer of the legacy stations that an error has occurred. The time when the PHY layer notifies the MAC layer of the legacy stations that an error has occurred coincides with the time when EPP information included in the data frame expires. Then the MAC layer of the legacy stations stands by for a predetermined amount of time corresponding to an extended interframe space (EIFS), whereas a plurality of HT stations stand by only for a predetermined amount of time corresponding to a distributed coordination function (DCF) interframe space (DIFS) and then contend for the use of the medium. Here, an EIFS is equal to the sum of a short interframe space (SIFS), the time (hereinafter referred to as the ACK reception time) required to receive an ACK frame in return for a data frame.
In other words, when an error occurs because the legacy stations cannot interpret a data frame having the HT format, the MAC layer of the legacy stations allows the legacy stations to begin to perform a backoff operation an EIFS (whose duration is 94 μs as prescribed in the IEEE 802.11a standard) after the reception of the data frame, whereas an MAC layer of the HT stations allows the HT stations to begin to perform a backoff operation a DIFS (whose duration is 34 μs as prescribed in the IEEE 802.11a standard) after the reception of the data frame. Therefore, the legacy stations cannot participate in the contention for the access to the medium under fair conditions. A CCA state of the legacy stations becomes idle after the reception of first HT data. However, since the time period specified in EPP information has not yet elapsed, the legacy stations do not notify the MAC layer that an error has occurred until a time-out period of the timers of the legacy stations elapses, regardless of whether the legacy stations receive an ACK frame having the HT format or an ACK frame having the legacy format.
In short, legacy stations can start an EIFS only after EPP information expires, i.e., after the reception of an ACK frame, whereas HT stations can start a DIFS after the reception of the ACK frame. Therefore, legacy stations are disadvantageous to HT stations when participating in contention for the use of a medium.